Low-cycle fatigue cracks of titanium alloy aerospace components are easily underestimated by conventional linear ultrasonic inspection technology, which is a potential safety hazard for aircraft service. In the research, the modulation nonlinear ultrasonic detection and quantitative technology of fatigue cracks in titanium alloys were carried out. A combination of the linear and the nonlinear ultrasonic quantitative method was proposed for the fatigue cracks in titanium alloys. Firstly, the fatigue crack is quantitatively detected by the metallographic method and the ultrasonic phased array technology. Secondly, a modulation nonlinear ultrasonic detection system and a sensor arrangement method are designed for strong anti-interference ability. Finally, a modulation non-linear ultrasonic quantitative method for fatigue cracks is proposed based on the analysis of the sound field distribution, and the quantitative detection results are compared with ultrasonic phased array detection technology. The research results indicate that the length of the macro-crack measured by the ultrasonic phased array method is 12 mm, while the micro-crack measured by the nonlinear ultrasonic method extends to a distance of 20 mm from the end of the narrow groove. Thus, the non-linear modulation detection technology has more significant advantages in detecting micro-cracks and closed cracks.